Which of these numbers is prime? ${46,\ 57,\ 61,\ 63,\ 65}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 46 are 1, 2, 23, and 46. The factors of 57 are 1, 3, 19, and 57. The factors of 61 are 1 and 61. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 65 are 1, 5, 13, and 65. Thus, 61 is a prime number.